It is known that numerous aircraft are equipped with a collision alert device making it possible to emit at least one alert signal in the event of a risk of the aircraft colliding with the terrain. Concerning the alert device, it may in particular be a device of TAWS (“Terrain Awareness and Warning System”) type, of EGPWS (“Enhanced Ground Proximity Warning System”) type or of GCAS (“Ground Collision Avoidance System”) type.
More precisely, such a TAWS collision alert device is generally capable of emitting the following various types of alerts, the triggerings of which are based on different algorithms and sources of information:                a first type of alerts, called “reactive” alerts, the triggering of which is based mainly on information provided by a radioaltimeter of the aircraft. There exist various categories of reactive alerts, designated by “Mode 1”, “independent Mode 1”, “Mode 2”, etc., which all detect potential risks of collision of the aircraft with the terrain, but the triggering of which relies on information provided by different measurement chains. Thus, the triggering of the Mode 1 reactive alert depends on the height of the aircraft above the terrain as determined by the radioaltimeter of the aircraft, and also on the vertical speed of the aircraft as provided by the onboard inertial platforms. On the other hand, the triggering of the independent Mode 1 reactive alert depends, on the one hand, on the height of the aircraft above the terrain as determined by onboard GPS (“Global Positioning System”) positioning means and also on a terrain database giving the altitude of the terrain and, on the other hand, on the vertical speed of the aircraft as provided by said GPS means. Moreover, the triggering of the Mode 2 reactive alert is dependent on the height of the aircraft above the terrain as determined by the radioaltimeter and also on the temporal derivative of this height;        a second type of alerts, called “predictive” alerts, the triggering of which relies on information contained in one or more databases (for example of terrain, obstacle, airport runway type, etc.). Predictive alerts such as these are for example called FLTA (“Forward Looking Terrain Alerting”) alerts when the database used is of terrain type, or else PDA (“Premature Descent Alerting”) alerts when the database used is of airport runway type.        
A terrain avoidance system for a civilian airplane equipped with a collision alert device of TAWS type is known through the document FR-2883403. This system automatically triggers a terrain avoidance maneuver when the collision alert device emits an alert (reactive or predictive) to signal that the airplane risks colliding with the terrain if it maintains its current flight characteristics. Thus, as soon as an alert is emitted, the system acts automatically (without pilot intervention) on the trajectory of the airplane, so as to improve the situation of the latter with respect to the terrain.
However, the reactive alerts being emitted after analyzing information provided mainly by the radioaltimeter of the airplane which can only detect the altitude of the terrain under the airplane, it may happen that some of these emitted reactive alerts do not allow genuine anticipation of the risks of the airplane colliding with the relief of the terrain situated ahead of it.
Furthermore, the predictive alerts are, for their part, dependent on the reliability and the precision of the databases used to trigger them. Hence, it may transpire that errors are present in these databases, sometimes giving rise to the emission of untimely predictive alerts, whereas in reality there is no substantiated danger of the airplane colliding with the terrain.
Moreover, certain terrain geometries are such that they do not bring about the emission of predictive alerts by the TAWS collision alert device. Thus, although the situation of the airplane with respect to the terrain so requires, it may transpire that no terrain avoidance maneuver is triggered if the triggering of such a maneuver relies only on the emission of a predictive alert.